Reports have appeared concerning the antitumor effect of poly I:C in vivo in animal tumor systems. Interferon induced by the double-stranded ribonucleic acid is thought to be important for the antitumor response in some experiments. The ability of a double-helix to induce interferon has been directly correlated with the stability of the helix. Recently synthesized ribonucleic acids containing antitumor antimetabolites form double-helicies with a range of stabilities (as measured by Tm). The effect of replacing normal oxo-bases with thio-analogs and 6-chloropurine upon the structure of double-helicies which incorporate the synthetic ribonucleic acids will be studied by using nuclear magnetic resonance (nmr) spectroscopy. The structural perturbations will be correlated with hydrogen bonding, stacking, and "looping out" interactions. The changes in structure and stability will be related to known antitumor and interferon inducing properties.